In nuclear power stations comprising a pressurized water reactor, the steam produced in the steam generators has a moisture content which is relatively low but nonetheless excessive for enabling this steam to be used in the turbines, without intermediate drying.
Steam generators of pressurized water reactors therefore comprise, in their upper part in the outlet path of the steam, drying devices which are of two types and are disposed in succession in the path of the steam. First of all, the steam enters centrifugal separators where it is driven in rotation in a helix, the droplets of water contained in the steam separating from the latter by the centrifugal effect before being trapped on a cylindrical wall which ensures the return of the water by gravity to the feed zone of the steam generator.
At the outlet of the centrifugal separators, the steam undergoes a second drying, termed fine drying, in baffle separators which produce sudden successive modifications in the direction of flow of the steam. The known baffle separators, which have a very high efficiency, comprise troughs of vertical positions associated with the baffles so as to recover the water separated from the steam. These troughs have hollow cross-sectional shapes in horizontal planes whose cavity faces toward the incident current of the steam so that the droplets of water separated from the steam on the walls of the baffles are stopped by the troughs, collected and conveyed by gravity to the lower part of the separators and then to the feed zone of the steam generator through drain conduits.
Although these troughs substantially increase the efficiency of the baffle driers, they have drawbacks which render their use more difficult.
First of all, these troughs interposed in the path of the steam increase the pressure drop in the drier. Further, they create a zone in which the steam is under overpressure just upstream of the liquid film constituted by the droplets collected in the troughs. In this overpressure zone, the current of steam and the droplets of water have a practically zero velocity, so that water particles are in equilibrium with a steam under overpressure. These particular conditions do not facilitate the elimination of the water by gravity flow.
Moreover, the current of steam is rising during the entire travel through the driers which are disposed in succession one above the other. This rising current entrains the droplets of water upwardly into zones located upstream of the troughs and in counter-current relative to the liquid film which retards its gravity flow.
The construction of baffle driers has been proposed in the form of assemblies constituted by a perforated vertical panel contacted by the incident current of steam which, after having travelled through the perforated panel, reaches a zone enclosing the baffles and the water-recovery troughs. The perforated panel, constituting the inlet wall of the drier, creates a pressure drop in the flow of the steam and deviates this current of steam downwardly. There is used, for example, as a perforated steam inlet panel a sheet of metal comprising orifices having a dimension and an arrangement which are chosen to modify substantially the flow of the steam without excessively decreasing the pressure drop. However, such deflecting devices have a very limited efficiency.
The use of panels has also been proposed constituted by a frame in which are placed series of parallel strips which are downwardly inclined in the direction of flow of the steam, of the venetian blind type. Such panels may have high efficiency as concerns the deflection of the current of steam, but they are delicate to construct when they are intended to come into contact with steam at very high temperature circulating at very high velocity. Whatever the method of fabrication of these panels, great strength and durability cannot be guaranteed.